1. Field of the Invention
The present invention relates to a light source wavelength control apparatus for use in a wavelength division multiplexing (WDM) optical telecommunication system.
2. Description of the Related Art
Recently, a signal generation part of a WDM optical transmission apparatus is equipped with a tunable laser diode (LD) capable of varying an output wavelength. A conventional tunable LD has been mainly of a type changing a wavelength by varying temperature of a peltiert device which maintains an LD temperature at constant, which is one varying a wavelength by a change of a refraction index of an active layer having a narrow range of variable wavelength, i.e., 2 to 4 nm.
However, recently becoming a mainstream is one capable of varying a wavelength fully in a C band range (or in an L band range) by controlling an external resonator attached externally to an LD element, thereby making a resonating optical path variable.
In a WDM apparatus, it is necessary to set a wavelength for each of the same tunable LDs equipped on individual transmitters at the time of operating it in order to obtain a desired wavelength. Conventionally performed method has been for setting a wavelength manually or recognizing an apparatus slot position on which the transmitter itself is mounted by the apparatus itself and setting a wavelength for the apparatus itself.
FIG. 1 is a diagram illustrating a conventional WDM optical transmission apparatus.
Inserted into a housing 10 are panels equipped with various functions, and the panels are interconnected by optical cables or electric wirings. Equipped on the left half of the housing 10 is a plurality of transmitter units including tunable LDs. A method for setting a wavelength for each of the transmitter units employs a method of a manual setup or of having each transmitter unit automatically recognize as to which slot the apparatus itself has been inserted in and set a wavelength automatically.
A manual setup, however, needs a very large amount of work when there are a large number of wavelengths. A method of an automatic setup by recognizing a slot position is an effective method for a configuration having only one shelf, whereas, in the case that there is a plurality of mounting shelf of transmitters for a multiple wavelength WDM telecommunication system, requiring a recognition as to which a wavelength-use slot to which the apparatus itself is inserted after performing an information exchange between shelves because there is a necessity of recognizing a type of a shelf in addition to a slot position, thus necessitating a complex control sequence.
FIG. 2 is a conceptual diagram of a configuration of a transmitter for use in a multiple wavelength WDM telecommunication system.
A single transmitter housing shelf is capable of housing a plurality of light sources, of which the number is up to tens at most. Therefore, a single shelf is not capable of accommodating all light sources for a recent multiple wavelength WDM telecommunications which multiplexes no less than hundred wavelengths, thus needing to furnish a plurality of transmitter housing shelves, multiplex the lights of respective wavelengths output from these shelves by a multiple wavelength multiplexing and division plus amplification apparatus 11 for sending them out to telecommunication paths.
Furthermore, this method is faced with a difficulty in recognizing an existence of a mistake if there is a wrong connection between a physical slot for mounting a transmitter and a module for multiplexing the lights, and, further, an increased difficulty in identifying a wrong connection point with the number of optical fibers which is proportionate to the number of wavelengths.
A transmission wave control method for a conventional WDM optical transmission system includes a patent document 1 which has disclosed a technique for controlling an output wavelength of a transmitter by detecting a return light back to the transmitter.                [Patent document 1] Japanese Patent Application Publication No. 2005-277686        
FIG. 3 is a diagram describing a problem associated with the conventional technique.
Individual transmitters 15-1 through 15-n respectively output wavelengths λ1 through λn. Since the respective output of the individual transmitters 15-1 through 15-n need to be multiplexed for generating a WDM signal, an optical multiplexer 16 is equipped. Individual input ports P1 though Pn of the optical multiplexer 16 include filter characteristics so as not to output a light except for that of a predetermined wavelength corresponding to each of the input ports.
Referring to FIG. 3, since the transmitters 15-n-1 and 15-n input lights of correct wavelength to ports 15-n-1 and P15-n of the optical multiplexer 16, they are multiplexed with other wavelength light to be output as a WDM signal.
Meanwhile, in the transmitters 15-1 and 15-2, the ports P1 and P2 to be connected are interchanged due to a wrong connection. The light of a wavelength λ1 of the transmitter 15-1 is input to the port P2 which receives only a wavelength λ2, while the light of a wavelength λ2 is input to the port P1 which receives only a wavelength λ1. In this case, the lights of the transmitters 15-1 and 15-2 are not multiplexed correctly at the optical multiplexer 16, and therefore not output as a WDM light because of the filter characteristics of the input ports. Consequently, the WDM signal is an optical multiplexed signal missing the lights of wavelengths λ1 and λ2.
In such a case, the problem can be solved by correcting the connection of the transmitters 15-1 and 15-2, the actual number of transmitters, however, counts up to anywhere from tens to a hundred and tens, with the same number of optical cables accompanying, making humanly very difficult to sort out a wrong connection manually and correct the misconnection.